I sold my CR-10S, so I will not be doing any more development / revisions to these file.

Modeled for my Creality CR-10S, 300x300x400.
Most frame stiffener brace kits that I see use 8mm all-thread or specialty tube and glue in inserts. I live in a small town in USA and nobody stocks metric all-thread. So I developed this kit using 5/16-18 unc all-thread. Also you will notice that I hide both hex nuts inside the printed parts for a cleaner look. The only consideration you need while assembling each side is if your just a faction off with your rod length, you will need to remove of of the hidden nuts and rotate it 1/6 of a turn. this will let you turn the perfect length of hitting the slots in the extruded rails perfectly. Well to 1/6th of a rotation.

The parts you need are:
(2) Upper Frame Brace (printed)
(2) Lower Frame Brace (printed)
(2) 5/16-18 unc all-thread x 24"
(8) 5/16-18 unc hex nuts (1/2" across flats)
(4) M3 x 20mm long socket head screws
(12) M3 x 10mm long socket head screws
(16) M3 hex nuts
(16) M3 hammer head nuts for 2020 aluminum extrusion.

You can print hammer head nuts using this file:
https://www.thingiverse.com/thing:2469130

Edit: I created "frame_brace_kit.stl" which is all 4 braces rotated in the correct orientation for printing. These models are identical to the original individual files.

I didn't have any 3M x 10mm long socket head screws, but i did have 12mm ones. and i modeled the flats up just enough for the head clearance and washers would not fit for spacers. So I printed out some quick 2mm spacers so the screws would not bottom out. lesson learned - count and measure your screws before you CAD up some models and print them.

This kit does slightly reduce max printing height.
Edit: It reduced my total height by 7mm. it also reduced my total width by 10mm when the gantry is over 310mm on the Z axis. I never print large max dimension boxes, so its not an issue for me.

I also ended up wrapping the All-thread in black electrical tape to give it a cleaner appearance.